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Light lattice that traps atoms could power networks of quantum computing

April 11, 2014

Scientists have developed a new method of trapping rubidium atoms (yellow circle) in a lattice of light (red bars), which could help the development of quantum computing (credit: Christine Daniloff/MIT)

Scientists at MIT and Harvard University have developed a new method for connecting atoms and light that could help in the development of powerful quantum computing systems.

The new technique allows researchers to couple a lone atom of rubidium, a metal, with a single photon, or light particle. This allows both the atom and photon to switch the quantum state of each other, providing a mechanism through which quantum-level computing operations could take place.

The scientists also believe their technique will allow them to increase the density or number of useful interactions occurring within a small space, thus scaling up the amount of quantum computing processing available.

That is, photons can have two polarization states, and interaction with the atom can change the photon from one state to another; conversely, interaction with the photon can change the atom’s phase, which is equivalent to changing the quantum state of the atom from its “ground” state to its “excited” state. In this way the atom-photon coupling can serve as a quantum switch to transmit information — the equivalent of a transistor in a classical computing system. And by placing many atoms within the same field of light, the researchers may be able to build networks that can process quantum information more effectively.

“You can now imagine having several atoms placed there, to make several of these devices — which are only a few hundred nanometers thick, 1,000 times thinner than a human hair — and couple them together to make them exchange information,” Vuletić adds.

Trapped in a photonic crystal cavity

Quantum computing could enable the rapid performance of calculations by taking advantage of the distinctive quantum-level properties of particles. Some particles can be in a condition of superposition, appearing to exist in two places at the same time. Particles in superposition, known as qubits, could thus contain more information than particles at classical scales, and allow for faster computing.

However, researchers are in the early stages of determining which materials best allow for quantum-scale computing. The MIT and Harvard researchers have been examining photons as a candidate, since photons rarely interact with each other. For this reason, an optical quantum computing system, using photons, could be harder to knock out of its delicate alignment. But since photons also rarely interact with other bits of matter, they are difficult to manipulate in the first place.

So in this case, the researchers used a laser to place a rubidium atom very close to the surface of a photonic crystal cavity, a structure of light. The atoms were placed no more than 100 or 200 nanometers — less than a wavelength of light — from the edge of the cavity. At such small distances, there is a strong attractive force between the atom and the surface of the light field, which the researchers used to trap the atom in place.

Other methods of producing a similar outcome have been considered before — such as, in effect, dropping atoms into the light and then finding and trapping them. But the researchers found that they had greater control over the particles this way.

“In some sense, it was a big surprise how simple this solution was compared to the different techniques you might envision of getting the atoms there,” Vuletić says.

The result is what he calls a “hybrid quantum system,” where individual atoms are coupled to microscopic fabricated devices, and in which atoms and photons can be controlled in productive ways. The researchers also found that the new device serves as a kind of router separating photons from each other.

Funding for the research was provided in part by the National Science Foundation, the MIT-Harvard Center for Ultracold Atoms, the Natural Sciences and Engineering Research Council of Canada, the Air Force Office of Scientific Research, and the Packard Foundation.

Abstract of Nature paper

By analogy to transistors in classical electronic circuits, quantum optical switches are important elements of quantum circuits and quantum networks. Operated at the fundamental limit where a single quantum of light or matter controls another field or material system, such a switch may enable applications such as long-distance quantum communication, distributed quantum information processing and metrology, and the exploration of novel quantum states of matter. Here, by strongly coupling a photon to a single atom trapped in the near field of a nanoscale photonic crystal cavity, we realize a system in which a single atom switches the phase of a photon and a single photon modifies the atom’s phase. We experimentally demonstrate an atom-induced optical phase shift that is nonlinear at the two-photon level, a photon number router that separates individual photons and photon pairs into different output modes, and a single-photon switch in which a single ‘gate’ photon controls the propagation of a subsequent probe field. These techniques pave the way to integrated quantum nanophotonic networks involving multiple atomic nodes connected by guided light.